Internet DRAFT - draft-bormann-cbor-cde
draft-bormann-cbor-cde
CBOR C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Best Current Practice 5 November 2023
Expires: 8 May 2024
CBOR Common Deterministic Encoding (CDE)
draft-bormann-cbor-cde-00
Abstract
CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in
its Section 4.2, providing some flexibility for application specific
decisions. To facilitate Deterministic Encoding to be offered as a
selectable feature of generic encoders, the present document defines
a CBOR Common Deterministic Encoding (CDE) Profile that can be shared
by a large set of applications with potentially diverging detailed
requirements.
This document also introduces the concept of Application Profiles,
which are layered on top of the CBOR CDE Profile and can address more
application specific requirements. To demonstrate how Application
Profiles can be built on the CDE, a companion document defines the
application profile "dCBOR".
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-cbor-cde/.
Discussion of this document takes place on the Concise Binary Object
Representation Maintenance and Extensions (CBOR) Working Group
mailing list (mailto:cbor@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at
https://www.ietf.org/mailman/listinfo/cbor/.
Source for this draft and an issue tracker can be found at
https://github.com/cabo/det.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Bormann Expires 8 May 2024 [Page 1]
Internet-Draft CBOR CDE November 2023
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 May 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. CBOR Common Deterministic Encoding Profile (CDE) . . . . . . 3
3. Application Profiles . . . . . . . . . . . . . . . . . . . . 5
4. CDDL support . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
Bormann Expires 8 May 2024 [Page 2]
Internet-Draft CBOR CDE November 2023
1. Introduction
CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in
its Section 4.2, providing some flexibility for application specific
decisions. To facilitate Deterministic Encoding to be offered as a
selectable feature of generic encoders, the present document defines
a CBOR Common Deterministic Encoding (CDE) Profile that can be shared
by a large set of applications with potentially diverging detailed
requirements.
This document also introduces the concept of Application Profiles,
which are layered on top of the CBOR CDE Profile and can address more
application specific requirements. To demonstrate how Application
Profiles can be built on the CDE, a companion document defines the
application profile "dCBOR".
1.1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. CBOR Common Deterministic Encoding Profile (CDE)
This specification defines the _CBOR Common Deterministic Encoding
Profile_ (CDE) based on the _Core Deterministic Encoding
Requirements_ defined for CBOR in Section 4.2.1 of [STD94].
In many cases, CBOR provides more than one way to encode a data item,
but also provides a recommendation for a _Preferred Encoding_. The
_CoRE Deterministic Encoding Requirements_ generally pick the
preferred encodings as mandatory; they also pick additional choices
such as definite-length encoding. Finally, it defines a map ordering
based on lexicographic ordering of the (deterministically) encoded
map keys.
Note that this specific set of requirements is elective — in
principle, other variants of deterministic encoding can be defined
(and have been, now being phased out slowly, as detailed in
Section 4.2.3 of [STD94]). In many applications of CBOR today,
deterministic encoding is not used at all, as its restriction of
choices can create some additional performance cost and code
complexity.
Bormann Expires 8 May 2024 [Page 3]
Internet-Draft CBOR CDE November 2023
[STD94]'s core requirements are designed to provide well-understood
and easy-to-implement rules while maximizing coverage, i.e., the
subset of CBOR data items that are fully specified by these rules,
and also placing minimal burden on implementations.
Section 4.2.2 of [STD94] picks up on the interaction of extensibility
(CBOR tags) and deterministic encoding. CBOR itself uses some tags
to increase the range of its basic generic data types, e.g., tags 2/3
extend the range of basic major types 0/1 in a seamless way.
Section 4.2.2 of [STD94] recommends handling this transition the same
way as with the transition between different integer representation
lengths in the basic generic data model, i.e., by mandating the
Preferred Encoding (Section 3.4.3 of [STD94]).
1. The CBOR Common Deterministic Encoding Profile (CDE) turns this
recommendation into a mandate: Integers that can be represented
by basic major type 0 and 1 are encoded using the deterministic
encoding defined for them, and integers outside this range are
encoded using the preferred serialization (Section 3.4.3 of
[STD94]) of tag 2 and 3 (i.e., no leading zero bytes).
Most tags capture more specific application semantics and therefore
may be harder to define a deterministic encoding for. While the
deterministic encoding of their tag internals is often covered by the
_Core Deterministic Encoding Requirements_, the mapping of diverging
platform application data types on the tag contents may be hard to do
in a deterministic way; see Section 3.2 of [I-D.bormann-cbor-det] for
more explanation as well as examples. As the CDE would continually
need to address additional issues raised by the registration of new
tags, this specification RECOMMENDS that new tag registrations
address deterministic encoding in the context of this Profile.
A particularly difficult field to obtain deterministic encoding for
is floating point numbers, partially because they themselves are
often obtained from processes that are not entirely deterministic
between platforms. See Section 3.2.2 of [I-D.bormann-cbor-det] for
more details. Section 4.2.2 of [STD94] presents a number of choices,
which need to be made to obtain a CBOR Common Deterministic Encoding
Profile (CDE). Specifically, CDE specifies (in the order of the
bullet list at the end of Section 4.2.2 of [STD94]):
2. Besides the mandated use of preferred encoding, there is no
further specific action for the two different zero values, e.g.,
an encoder that is asked by an application to represent a
negative floating point zero will generate 0xf98000.
Bormann Expires 8 May 2024 [Page 4]
Internet-Draft CBOR CDE November 2023
3. There is no attempt to mix integers and floating point numbers,
i.e., all floating point values are encoded as the preferred
floating-point representation that accurately represents the
value, independent of whether the floating point value is,
mathematically, an integral value (choice 2 of the second
bullet).
4. There is no special handling of NaN values, except that the
preferred encoding rules also apply to NaNs with payloads, using
the canonical encoding of NaNs as defined in [IEEE754].
Typically, most applications that employ NaNs in their storage
and communication interfaces will only use the NaN with payload
0, which encodes as 0xf97e00.
5. There is no special handling of subnormal values.
6. The CBOR Common Deterministic Encoding Profile does not presume
equivalence of floating point values with other representation
(e.g., tag 4/5) with basic floating point values.
The main intent here is to preserve the basic generic data model, so
Application Profiles can make their own decisions within that data
model. E.g., an application profile can decide that it only ever
allows a single NaN value that would encoded as 0xf97e00, so a CDE
implementation focusing on this application profile would not need to
provide processing for other NaN values. Basing the definition of
both CDE and Application Profiles on the generic data model of CBOR
also means that there is no effect on CDDL [RFC8610], except where
the data description documents encoding decision for byte strings
carrying embedded CBOR.
3. Application Profiles
While the CBOR Common Deterministic Encoding Profile (CDE) provides
for commonality between different applications of CBOR, it is useful
to further constrain the set of data items handled in a group of
applications (_exclusions_) and to define further mappings
(_reductions_) that help the applications in such a group get by with
the exclusions.
For example, the dCBOR Application Profile specifies the use of
Deterministic Encoding as defined in Section 4.2 of [STD94] (see also
[I-D.bormann-cbor-det] for more information) together with some
application-level rules. See [I-D.bormann-cbor-dcbor] for a
definition of the dCBOR Application Profile that makes use of CDE.
Bormann Expires 8 May 2024 [Page 5]
Internet-Draft CBOR CDE November 2023
In general, the application-level rules specified by an Application
Profile are based on the same CBOR Common Deterministic Encoding
Profile; they do not "fork" CBOR.
An Application Profile implementation produces well-formed,
deterministically encoded CBOR according to [STD94], and existing
generic CBOR decoders will therefore be able to decode it, including
those that check for Deterministic Encoding. Similarly, generic CBOR
encoders will be able to produce valid CBOR that can be processed by
Application Profile implementations, if handed Application Profile
conforming data model level information from an application.
Please note that the separation between standard CBOR processing and
the processing required by the Application Profile is a conceptual
one: Both Application Profile processing and standard CBOR processing
can be combined into a encoder/decoder specifically designed for the
Application Profile.
An Application Profile is intended to be used in conjunction with an
application, which typically will use a subset of the CBOR generic
data model, which in turn influences which subset of the application
profile is used. As a result, an Application Profile itself places
no direct requirement on what minimum subset of CBOR is implemented.
For instance, an application profile might define rules for the
processing of floating point values, but there is no requirement that
implementations of that Application Profile support floating point
numbers (or any other kind of number, such as arbitrary precision
integers or 64-bit negative integers) when they are used with
applications that do not use them.
4. CDDL support
[RFC8610] defines control operators to indicate that the contents of
a byte string carries a CBOR-encoded data item (.cbor) or a sequence
of CBOR-encoded data items (.cborseq).
CDDL specifications may want to specify that the data items should be
encoded in Common CBOR Deterministic Encoding. This specification
adds two CDDL control operators that can be used for this.
The control operators .cde and .cdeseq are exactly like .cbor and
.cborseq except that they also require the encoded data item(s) to be
in Common CBOR Deterministic Encoding.
For example, a byte string of embedded CBOR that is to be encoded
according to CDE can be formalized as:
leaf = #6.24(bytes .cde any)
Bormann Expires 8 May 2024 [Page 6]
Internet-Draft CBOR CDE November 2023
More importantly, if the encoded data item also needs to have a
specific structure, this can be expressed by the right hand side
(instead of using the most general CDDL type any here).
(Note that the ...seq control operator does not enable specifying
different deterministic encoding requirements for the elements of the
sequence. If a use case for such a feature becomes known, it could
be added.)
Obviously, Application Profiles can define similar control operators
that also embody the processing required by the Application Profile,
and are encouraged to do so.
5. Security Considerations
TODO Security
6. IANA Considerations
// RFC Editor: please replace RFCXXXX with the RFC number of this RFC
// and remove this note.
This document requests IANA to register the contents of Table 1 into
the registry "CDDL Control Operators" of [IANA.cddl]:
+=========+===========+
| Name | Reference |
+=========+===========+
| .cde | [RFCXXXX] |
+---------+-----------+
| .cdeseq | [RFCXXXX] |
+---------+-----------+
Table 1: New control
operators to be
registered
7. References
7.1. Normative References
[IANA.cddl]
IANA, "Concise Data Definition Language (CDDL)",
<https://www.iana.org/assignments/cddl>.
Bormann Expires 8 May 2024 [Page 7]
Internet-Draft CBOR CDE November 2023
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
<https://ieeexplore.ieee.org/document/8766229>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
7.2. Informative References
[I-D.bormann-cbor-dcbor]
Bormann, C., "Common CBOR Deterministic Encoding and
Application Profiles", Work in Progress, Internet-Draft,
draft-bormann-cbor-dcbor-03, 22 August 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
dcbor-03>.
[I-D.bormann-cbor-det]
Bormann, C., "CBOR: On Deterministic Encoding", Work in
Progress, Internet-Draft, draft-bormann-cbor-det-01, 9
August 2023, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-det-01>.
[I-D.mcnally-deterministic-cbor]
McNally, W. and C. Allen, "Gordian dCBOR: A Deterministic
CBOR Application Profile", Work in Progress, Internet-
Draft, draft-mcnally-deterministic-cbor-05, 8 August 2023,
<https://datatracker.ietf.org/doc/html/draft-mcnally-
deterministic-cbor-05>.
Bormann Expires 8 May 2024 [Page 8]
Internet-Draft CBOR CDE November 2023
Acknowledgments
An earlier version of this document was based on the work of Wolf
McNally and Christopher Allen as documented in
[I-D.mcnally-deterministic-cbor]; the parts directly based on this
are now separated out as the dCBOR Application Profile
[I-D.bormann-cbor-dcbor]. Nonetheless, we acknowledge that this work
has contributed greatly to shaping the concept of a CBOR Common
Deterministic Encoding and Application Profiles on top of that.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Bormann Expires 8 May 2024 [Page 9]
